Gas-fired, storage-type water heaters often include a combustion chamber and air plenum disposed below a water tank. A burner element, gas manifold tube, ignition source, thermocouple, and a pilot tube typically extend into the combustion chamber. When the temperature of the water in the tank falls below a set minimum, gas fuel is introduced into the combustion chamber through the gas manifold tube and burner element. This gas fuel is ignited by the pilot flame or other ignition source, and the flame is maintained around the burner element. Air is drawn into the plenum, and mixes with gas fuel to support combustion within the combustion chamber. The products of combustion typically flow through a flue or heat exchange tube in the water tank to heat the water by convection and conduction.
In some cases, a water heater may be positioned in an area that is also occupied by lawnmowers, chain saws, snow blowers, trimmers, and other equipment having a gasoline-powered internal combustion engine. In such cases, it is not uncommon that there be gasoline and other flammable substances (e.g., kerosene, diesel, turpentine, solvents, alcohol, propane, methane, and butane) present in the same area. Such flammable substances often emit flammable vapors.
If the flammable substances are mishandled, the flammable vapors may encounter an ignition source, such as the pilot flame or burner flame of a gas-fired water heater. As a result of the mishandling of flammable substances, the flammable vapors may ignite, and the flame may follow the flammable vapors to their source, causing an explosion and/or a fire. Proposed governmental regulations would require residential gas-fired water heaters to be flammable vapor resistant in order to help reduce the occurrence of such dangerous situations caused by the mishandling of flammable substances.